1. Field of the Invention
The present invention relates to a welding method in the manufacture of a spirally wound tube, as well as to a welding head for performing the method.
2. Description of the Relevant Art
Previously, welding of spirally wound tubes having a polyethylene hollow profile was performed in two steps. During a first rotation of a welding drum, whereby the hollow profile was provided around the welding drum, an inner seam was welded onto the closest previous winding of the hollow profile with a single welding keel; and during the following rotation, an outer seam was welded with a single extruder shoe.
A drawback of this previously used method is that it requires two extruders, and the controlling of the total welding operation is relatively difficult partially because the heating is carried out by means of hot-air insufflators, not integrated with the welding keel, so they have to be separately adjusted resulting in a very varying heating results, and partially because of great fluctuations in the welding pressure. This previously used welding method does not satisfy all the requirements for a good weld for a polyethylene, which especially concerns the outer seam.
A condition for a good weld seam in polyethylene is, besides cleanliness, the basic material being heated to correct temperature and the surfaces to be seamed together being pressed together under pressure.